1. Field of the Invention
The present invention relates to a method of controlling an electrically operated injection molding machine, and more particularly, is directed to a control of injection, dwell and back pressure.
2. Description of the Related Art
In a typical electrically operated injection molding machine, its screw is axially driven by a drive of a servomotor to control injection, dwell and back pressure thereof. The injection process among them generally undergoes a multi-stage control of injection speed in which its screw stroke is divided into a plurality of segments and the injection speed is assigned each of the segments. Furthermore, the conventional dwell process is subjected to a dwell control where an injection servomotor receives a movement instruction to bring a screw to an extremity of cylinder, and is driven in compliance with a set dwell speed instruction with its output torque being restricted to a set dwell. As a result, even though the screw is driven toward the injection direction by driving the servomotor with its output torque restricted, the amount of movement of the screw is relatively small due to the filling of a mold with a resin, and accordingly a positional deviation relative to the instructed position increases with time. Irrespective of the increase in the positional deviation, however, as the output torque of the servomotor is restricted to a set value, the servomotor is not permitted to produce an excessive torque beyond the set value, to consequently subject the resin to a set dwell pressure corresponding to a restricted torque value. In this manner, the known electrically operated injection molding machine undergoes a dwell control through the restriction of the output torque of the injection servomotor.
Moreover, the conventional metering process employs a pressure control of a type in which the injection servomotor rotates the screw with its output torque set to present a predetermined back pressure, during which the resin melts to increase the resin pressure, and the screw retreats when the resin pressure is larger than the torque produced by the injection servomotor, to consequently subject the resin to the back pressure which has been set. Also known is a back pressure control of a type in which the screw is caused to retreat by a predetermined amount every time the resin pressure surpasses the given back pressure, to thereby confer the given back pressure on the resin.
As described hereinbefore, the conventional electrically operated injection molding machine requires a changeover from the screw position control (or speed control) to the pressure control when changing over from the injection process (or injection speed control process) to the dwell process. Furthermore, it is necessary to change over from the pressure control to the position control if the back pressure at metering is controlled based on the screw position at the time of changeover from the dwell process to the metering process.
Additionally, it is very common in the dwell control to segment the dwell pressure into several stages for the control thereof. Accordingly, in the case where the dwell control is performed with the restricted output torque of the injection servomotor, the torque produced by the servomotor balances with the resin pressure for the dwelling. As a result, if the dwell pressure is changed from a larger value to a smaller one, the valance is lost and the resin pressure becomes remarkably larger than the pressure corresponding to the restricted output torque of the servomotor, whereby the screw may be pushed away and forced to retreat to a great extent. This disadvantageously leads to an improper dwell control.
On the contrary, providing that the dwell pressure is changed from a smaller value to a larger one, the output torque of the servomotor is enhanced in response to the changeover, but the screw is allowed to move only at the predetermined speed in accordance with the dwell speed instruction. Hence, there arises a time lag to reach a newly set dwell pressure, which disadvantageously results in a dwell control having poor response characteristic.